macromoleculesproteins

Proteins are large, diverse biomolecules and a principal class of macromolecules in biology. They are polymers built from 20 standard amino acids linked by peptide bonds, forming linear polypeptide chains that fold into specific three-dimensional shapes. The amino acid sequence, encoded in genes, dictates how a protein folds and, therefore, its function and properties. Proteins perform a broad spectrum of cellular tasks and are essential for life.

Proteins have four levels of structure. The primary structure is the linear sequence of amino acids. Secondary structures arise from hydrogen bonding, forming elements such as alpha helices and beta sheets. The tertiary structure is the overall three-dimensional fold of a single polypeptide, stabilized by interactions among side chains. If a protein consists of multiple polypeptide chains, its overall arrangement is the quaternary structure.

Proteins are synthesized through gene expression. Messenger RNA is translated by ribosomes to produce a nascent polypeptide, which folds into its native structure with the help of molecular chaperones and, often, cofactors or prosthetic groups. After synthesis, many proteins undergo post-translational modifications that can alter activity, localization, or stability.

Proteins have diverse functions. They act as enzymes to catalyze biochemical reactions, provide structural support, transport and store molecules, regulate cellular processes, mediate signaling and immune responses, and drive movement. Based on shape and properties, proteins are often categorized as globular (compact, soluble) or fibrous (elongated, structural).

Stability and function of proteins depend on environmental conditions such as temperature, pH, and solvent exposure. Denaturation or misfolding can impair activity and is linked to diseases. Modern methods such as sequencing, X-ray crystallography, NMR, and cryo-electron microscopy help characterize protein structure and function.